In the better designs, by lifting a perforated bucket out through the access cover, tipping it and returning it. The solids never have to be pumped, and the chamber is not entered.

Larger interceptors settle into an open chamber instead, and are emptied by a vacuum tanker. That works, but it is a scheduled contractor visit rather than something site staff can do.

Bucket capacity is therefore chosen against how often somebody is realistically going to lift it. A bucket sized so it is too heavy when full will be emptied late or not at all, whatever the schedule says.

To keep abrasive sediment out of equipment that separation depends on. A coalescing element or a skimmer buried in grit stops working, and cleaning it is far more disruptive than emptying a bucket.

It also protects the volume of the downstream unit. Silt settling in a grease or oil separator occupies retention capacity that the separator needs for its actual duty, shortening the interval between services.

Splitting the two functions means each is serviced on its own cycle: frequent and simple for the silt, less frequent and specialist for the separator.

Coarse material settles readily; fine silt and clay largely do not. Settlement depends on particle density and size against the upward drag of the flow, so heavier and larger particles fall out within the residence time and the finest fraction stays in suspension.

Increasing the chamber volume lowers the flow velocity and captures a finer fraction, which is why a larger unit is not only a longer service interval but genuinely better separation.

For very fine material a settlement chamber is the wrong tool. Filtration or a treatment stage is needed, and expecting a gravity interceptor to polish out clay leads to a unit that appears to work and does not.

Anywhere runoff carries solids: vehicle wash bays, construction site exits, quarry and aggregate yards, agricultural hardstandings, and the drainage of any area regularly swept or scraped.

Wash bays are the clearest case, because the water is deliberately used to carry material off vehicles and the sediment load is continuous and predictable.

They are also fitted inside buildings on floor gullies in food production and workshops, where the collected solids are process debris rather than grit but the mechanism is identical.

By the traffic passing over it, using the same load rating scheme that applies to surface drainage generally: pedestrian, car park, road or heavy industrial duty.

The cover also has to be large enough to lift the bucket out through, which is a constraint people meet only when the first service is attempted. A cover sized to the chamber rather than to the bucket makes routine emptying impossible.

Where the interceptor sits in a trafficked area, a locking or sealed cover is usual, both to stop it being displaced by a wheel and to keep odour and debris out between services.

It depends what is downstream. Where the interceptor connects to a foul drain, a trap seal is needed to stop sewer gas rising through the chamber and the gully above it.

On a surface water system discharging to a soakaway or watercourse there is no sewer gas to exclude, and an unsealed chamber is common and simpler to clean.

Where a seal is fitted it needs to be maintained by regular flow. An interceptor on a seasonal wash bay can dry out over a quiet period and lose its seal, which is the usual explanation for an intermittent smell in an otherwise sound system.

The settled bed grows upward until it reaches the outlet, and from then on solids pass straight through to whatever the interceptor was protecting.

That failure is silent from the surface. Water still drains, the cover still looks the same, and the first evidence is usually a worn pump, a blinded coalescer or a blocked run downstream.

Compacted sediment also becomes progressively harder to remove. Material that would have lifted out in a bucket at three months may need jetting and vacuum extraction at eighteen.